Biomedical measurements, such as electrocardiogram (ECG) and electroencephalogram (EEG) measurements, are typically made by affixing two or more electrodes to a patient. The electrodes are electrically connected to an instrumentation amplifier which measures the electrical potential between the electrodes. One source of measurement error is the common mode voltage applied to the amplifier. The common mode voltage is induced on the patient by nearby power conductors, static charge and other electrical sources. The common mode voltage is applied equally to both amplifier inputs and, for an ideal amplifier, has no effect on the amplifier output. While practical amplifiers attenuate common mode voltages by 60 dB or more, an output voltage is produced, thereby introducing a measurement error.
One prior art approach to reducing the errors that result from common mode voltage is to connect the patient to earth ground. This approach has been discontinued for safety reasons. Reduction of common mode voltage errors in biopotential amplifiers is discussed by B. B. Winter et al in "Reduction Of Interference Due to Common Mode Voltage in Biopotential Amplifiers", IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 1, January 1983, pages 58-62.
Another prior art approach, often called a "right leg drive circuit", is exemplified by the circuit shown in FIG. 1. Electrodes 10 and 12, represented in FIG. 1 as resistors, are attached to a patient 14. The electrodes 10 and 12 are connected to inputs 16 and 18, respectively, of a differential amplifier 20 (including operational amplifiers 25 and 26, and resistors 27, 28 and 29). A common mode voltage applied to the inputs of amplifier 20 is sensed by resistors 22 and 24 connected between the outputs of amplifier 20. A sensed voltage representative of the common mode voltage is applied to the input of an integrating amplifier 30. The output of amplifier 30 is connected to a third electrode 32 attached to patient 14. Typically, the third electrode 32 is connected to the patient's right leg for ECG and to the reference electrode for EEG. The integrating amplifier 30 completes a feedback loop which supplies current to the patient 14 to reduce the common mode input voltage change relative to the power supply voltages for amplifier 20. Further details regarding the right leg drive circuit are described by B. B. Winter et al in "Driven-Right-Leg Circuit Design", IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 1, January 1983, pages 62-66. The major limitations of the right leg drive circuit are that a third connection to the patient is required and that the 60 Hz common mode voltage attenuation is typically only about 100 dB.
Yet another prior art circuit is disclosed in U.S. Pat. No. 4,191,195 issued Mar. 4, 1980 to Miller. A voltage representative of the common mode voltage is amplified and is applied to a capacitor connected to circuit ground. The disclosed circuit imprecisely attenuates the common mode signal by imprecisely raising the ground referenced common mode impedance.
Still another prior art circuit is a modification of the right leg drive circuit shown in FIG. 1. A negative capacitance amplifier is connected between earth ground and the inputs to the instrumentation amplifier. The negative capacitance amplifier imprecisely attenuates the common mode signal by imprecisely raising the ground referenced common mode impedance.